The introduction of new services in wireless networks puts a premium on spectral efficiency and coverage in cellular radio networks. Cellular networks have come a long way since the analog voice telephone systems of the mid 1980s, such as the Advanced Mobile Phone Service (AMPS) or the Nordic Mobile Telephone (NMT) System. The 1990s saw the introduction of second generation digital cellular technologies such as the Global System for Mobile Communication (GSM) and packet data systems such as the General Packet Radio Service (GPRS) and their evolved third generation versions, Enhanced Data Rates for Global Evolution (EDGE) and Enhanced GPRS (EGPRS), respectively. The need for higher bandwidths and data rates also led to standardization of the Universal Mobile Telephone Service (UMTS). Third Generation (3G) standardization of GSM/EDGE and UMTS has been carried out in the 3GPP, whose focus has been on specifying a High Speed Packet Access (HSPA) service for WCDMA and Orthogonal Frequency Division Multiplexing (OFDM)-based evolution of 3G in a standard known as Long Term Evolution (LTE).
The performance of a wireless network is evaluated using several figures of merit, such as data rates, coverage and capacity. Capacity is interpreted in two different ways depending on the context of use. The classical definition of capacity is obtained from Shannon's a theoretical maximum rate of transmission at which communication can proceed over a noisy channel with arbitrarily low error probabilities. If the channel has no feedback from the receiver back to the transmitter, the figure of merit obtained is the open-loop capacity, while a channel with feedback may be used to derive a closed loop capacity. In the context of this application, the term capacity refers to the Shannon capacity of the communication channel. Cellular systems may also define capacity in terms of measures such as erlang capacity per cell referring to the number of call-hours of conversation for telephony, or measures of spectral efficiency identifying the number of bits of information transferred to the receiver per second of time per Hz of consumed bandwidth per cell (b/s/Hz/cell).
Using multiple antenna technologies improves data rates, coverage, and capacity. Multiple antenna technologies may employ Space-time Transmit Diversity (STTD), beam-forming, Spatial Multiplexing (SM), or Multiple-Input Multiple Output (MIMO). Another multiple antenna technology called Per-Antenna Rate Control (PARC) has been proposed for use in base station transmitters.
The PARC scheme is based on a combination transmit/receive architecture that performs independent coding of multiple downlink antenna streams transmitted at different rates, which is then complemented by the application of successive interference cancellation (SIC) at the receiver. PARC requires feedback from the receiving mobile terminal or station of the per-antenna data rates that are related to the signal-to-interference-plus-noise ratio (SINR) at each stage of the SIC. It has been shown that the PARC scheme can achieve an open-loop Shannon capacity of the MIMO channel in a flat-fading environment. Closed loop Shannon capacity is greater than open loop Shannon capacity due to the availability of channel state information from the receiver. In frequency selective MIMO channels, the performance of the PARC scheme suffers with respect to the capacity-achieved using a closed-loop transmission scheme.
Selective-Per-Antenna-Rate-Control (S-PARC) is an extension of PARC. The S-PARC scheme can achieve rates that are between the open loop and closed loop capacity. While the PARC scheme simultaneously transmits separately encoded streams at different rates from all available transmit antennas, the S-PARC scheme improves performance by adaptively selecting a subset of the available transmit antennas from which to transmit a reduced number of data streams. This maximizes the data rate transmitted while simultaneously limiting the self-interference between streams. The selection of the best antennas for transmission is determined by maximizing the sum rate of the transmitted data streams over the possible antenna combinations of the subset of antennas. Essentially, when radio channel conditions are poor, fewer data streams are transmitted. As conditions improve, more data streams are transmitted. By limiting the number of transmitted data streams to what the channel supports, excessive self-interference is avoided. Furthermore, when the number of transmitted streams is limited, antenna selection exploits available transmit diversity.
The PARC and S-PARC approaches can be used for multiple antenna transmissions on the downlink, and work rather well in enhancing rate, coverage, and capacity when transmitting data downlink from a base station to multiple mobiles in the system. But the inventors recognized that there is a need for similar enhancements for uplink communications from mobiles to the radio network. Indeed, certain classes of applications, such as video telephony, video blogging, file transfer for peer-to-peer applications, etc., are some examples of uplink applications that would immediately benefit from enhanced transmission rate, coverage, or capacity. Although MIMO solutions proposed for the downlink are capable of enhancing the amount of data traffic being sent from the base station, they have limited applicability to uplink communications because mobile stations typically do not use more than one transmit antenna. The single antenna limitation is a direct result of the small size of the mobile station and limited transmitted power typically available. Even if it were possible to build a mobile station with multiple antennas, the channels from those antennas to one particular receive antenna on the base station may be correlated limiting the diversity gain on the uplink.